This invention relates to an improved process for preparing acrylic acid from propylene using first and second stage reactors, xe2x80x9ctandem reactorsxe2x80x9d and an improved process for purifying the acrylic acid. In particular, the invention relates to a tandem reactor process for preparing acrylic acid from propylene utilizing an increased concentration of propylene reactant thereby providing increased capacity and throughput.
The preparation of acrylic acid from propylene generally proceeds in a vapor phase two stage catalytic oxidation reaction. In the first stage propylene is oxidized in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to produce acrolein according to equation (I):
C3H6+O2xe2x86x92C2H3CHO+H2O+heatxe2x80x83xe2x80x83(I).
The acrolein is then oxidized, in a second stage, in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to form acrylic acid according to equation (II):
C2H3CHO+xc2xdO2xe2x86x92C2H3COOH+heatxe2x80x83xe2x80x83(II).
The two stage vapor phase catalytic oxidation of propylene to acrylic acid is generally performed using either tandem reactors wherein a separate reactor is utilized for each step, e.g., see the description in U.S. Pat. No. 4,873,368, or by utilizing one reactor to perform both steps, e.g., see the description in U.S. Pat. No. 4,526,783.
The acrylic acid prepared using such a vapor phase catalytic oxidation reaction is present in a mixed product gas exiting the second stage reactor. Generally, the mixed product gas is cooled and is contacted with an aqueous stream in an absorption tower, thereby providing an aqueous acrylic acid solution from which acrylic acid can be isolated and purified. The remainder of the product gasses, known as the absorber waste gas or absorber off-gas, is incinerated. Depending on the reactants feed gas composition, the absorber off-gas may contain inert gasses, O2, water vapor, CO, CO2, unreacted propylene, unreacted acrolein and/or acrylic acid.
The tandem reactor processes known in the art are useful, however there is a continuing need for a process which is more efficient. By more efficient is meant a process which provides more acrylic acid with the same or smaller equipment, or creates less waste. Generally, increasing the propylene was thought in the art to be an unsuitable mechanism for increasing throughput and efficiency in the process because of the dangers of flammability and run-away reactions. Consequently, the oxidation of propylene to acrylic acid is generally practiced in the art utilizing a propylene concentration in the reactant gas feed composition of between 4 and 7 volume percent of the total reactant feed composition (see for example col. 2, lines 42-46 of U.S. Pat. No. 4,873,368).
U.S. Pat. Nos. 4,365,087 and 4,873,368 have dealt with the problem of increasing process productivity/capacity by raising the propylene concentration level. The processes disclosed in these references used a tandem reactor process whereby either the temperature of the feed was limited ( less than 260xc2x0 C.), the oxygen to propylene molar ratio (1.1-2.0:1, preferably lower than 1.8) was kept low, additional oxygen and inert gas was fed to the second stage reactor, and the reaction was quenched somewhat before introduction to the second stage (""087) or the oxygen to propylene molar ratio (1.17-1.66:1) was even lower, additional oxygen and inert gas was fed to the second stage reactor, and the reaction was quenched somewhat before introduction to the second stage. The quenching was achieved through passing the products of the first stage reaction through a bed of a solid inactive material. Passing the products through this bed results in a pressure drop. The technique relied on two mechanisms for controlling the reaction at higher propylene concentrations:
(1) tightly controlling the temperature before entry into the first stage reactor and/or the second stage reactor; and
(2) limiting the amount of oxygen initially available to the first reactor for oxidation of propylene to acrolein and then adding more oxygen and diluent at the interstage before the second stage reactor so that the second reactor feed has a stoichiometrically sufficient amount of oxygen to allow suitable oxidation of acrolein to acrylic acid.
It generally is undesirable to require feeding additional oxygen to the second stage of the reaction because of, as with the first stage, the possibility of increased incidence of flammability problems and runaway reactions. It is also undesirable to have a pressure drop due to cooling through a solid bed because resultant pressure drops lead to reduced selectivity for acrolein or acrylic acid.
The present inventors have now discovered that with the tandem reactor system described herein it is possible to provide feeds to the reactors which contain a high concentration of propylene without the problems associated with the cited processes. Such high concentration feeds are accomplished without the need to utilize a lower oxygen:propylene feed ratio, and the consequent addition of oxygen and inert gas to the second stage to assure proper stoichiometry. Applicants have also avoided the pressure drop associated with cooling through a solid bed, by substituting a heat exchanger for cooling purposes.
Accordingly, a novel process for preparing acrylic acid from propylene is described herein wherein the following advantages are provided:
(1) downstream debottlenecking is realized through producing an aqueous acrylic acid stream in the absorber having a higher concentration of acrylic acid because less water is present;
(2) since there is less water condensed in the aqueous acrylic acid there is a reduction in the waste generated by the process; and
(3) there is a lower pressure drop in the reactors, due to increased feed composition, which offsets increased propylene partial pressure, thereby preventing lower acrylic acid selectivity resulting from higher propylene pressure.
In one aspect of the present invention, there is provided a process for the vapor phase oxidation of propylene to acrylic acid, comprising the steps of: (A) feeding a reactant composition comprising: (i) greater than 7 percent by volume propylene, (ii) oxygen, (iii) water vapor, and (iv) the remainder including a major amount of at least one inert gas, into a reactor; the reactor including a plurality of contact tubes, containing at least one catalyst, disposed in a shell, wherein the inside of the reactor shell contains at least one heat transfer zone through which a heat transfer medium passes and each contact tube comprises at least one reaction zone capable of effecting the preparation of acrolein from propylene, (B) contacting the reactant composition with the at least one reaction zone to form a mixed product gas comprising acrolein, (C) cooling the mixed product gas comprising acrolein in a heat exchanger, (D) feeding the mixed product gas comprising acrolein to a second reactor; the second reactor including a plurality of contact tubes, containing at least one catalyst, disposed in a shell, wherein the inside of the reactor shell contains at least one heat transfer zone through which a heat transfer medium passes and each contact tube comprises at least one reaction zone capable of effecting the preparation of acrylic acid from acrolein, and (E) contacting the reactant composition with the at least one reaction zone to form a mixed product gas comprising acrylic acid.